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LTM4602
APPLICATIONS INFORMATION
After the controller has been started and given adequate
time to charge up the output capacitor, C SS is used as a
short-circuit timer. After the RUN/SS pin charges above 4V,
if the output voltage falls below 75% of its regulated value,
then a short-circuit fault is assumed. A 1.8μA current then
begins discharging C SS . If the fault condition persists until
4V maximum latchoff threshold and overcome the 4μA
maximum discharge current. Figure 3 shows a conceptual
drawing of V RUN during start-up and short circuit.
V RUN/SS
4V
the RUN/SS pin drops to 3.5V, then the controller turns
off both power MOSFETs, shutting down the converter
permanently. The RUN/SS pin must be actively pulled
down to ground in order to restart operation.
1.5V
3V
SHORT-CIRCUIT
LATCH ARMED
3.5V
The overcurrent protection timer requires the soft-start
timing capacitor C SS be made large enough to guarantee
that the output regulation by the time C SS has reached the
SOFT-START
CLAMPING
OF I L RELEASED
OUTPUT
OVERLOAD
HAPPENS
SHORT-CIRCUIT
LATCHOFF
t
4V threshold. In general, this will depend upon the size of
the output capacitance, output voltage and load current
characteristic. A minimum external soft-start capacitor
V OUT
75%V O
can be estimated from:
C SS_EXT + 1000pF > C OUT ? V OUT (10 –3 [F / V S ])
Generally 0.1μF is more than suf?cient.
SWITCHING
STARTS
Figure 3. RUN/SS Pin Voltage During Startup and
Short-Circuit Protection
t
4602 F03
Since the load current is already limited by the current mode
control and current foldback circuitry during a short circuit,
overcurrent latchoff operation is NOT always needed or
desired, especially if the output has large capacitance or
the load draws high current during start up. The latchoff
feature can be overridden by a pull-up current greater than
V IN
R RUN/SS
V IN
LTM4602
RUN/SS
PGND SGND
5μA but less than 80μA to the RUN/SS pin. The additional
RECOMMENDED VALUES FOR R RUN/SS
current prevents the discharge of C SS during a fault and
V IN
R RUN/SS
also shortens the soft-start period. Using a resistor from
RUN/SS pin to V IN is a simple solution to defeat latchoff. Any
4.5V TO 5.5V
10.8V TO 13.8V
16V TO 20V
50k
150k
330k
4602 F04
pull-up network must be able to maintain RUN/SS above
Figure 4. Defeat Short-Circuit Latchoff with a Pull-Up
Resistor to V IN
4602fa
12
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